Liquid calories (especially in the form of sugar water) do not satiate somebody in the way e.g. fruits would (since fruit contains fiber and "fills" one up better). If one took a top-down approach to diet and planned out intake appropriately (as it sounds like you do) then it would certainly be easy enough to slot in some sugar water and still meet all of one's nutritional needs. The sad truth is that most people are not very mindful of what (and just as critically how much) they are eating, presumably due to poor education.
I would be inclined to speculate that in humans most of the benefit of reducing sugar intake is in the form of improved satiety when sugar is replaced with other foods. Sugar water and the like simply don't satiate in the way most other calorie sources do. That doesn't matter to a person who is mindful of how many calories they need to consume, but a person who is not educated or mindful of caloric and nutritional requirements might grab a glass of sugar water and later on (upon not being satiated) they might simply return for another one.
I would be inclined to speculate that in humans most of the benefit of reducing sugar intake is in the form of improved satiety when sugar is replaced with other foods. Sugar water and the like simply don't satiate in the way most other calorie sources do. That doesn't matter to a person who is mindful of how many calories they need to consume, but a person who is not educated or mindful of caloric and nutritional requirements might grab a glass of sugar water and later on (upon not being satiated) they might simply return for another one.